1) Field of the Invention
The invention relates generally to increasing the efficiency and frequency band of operation of all transducers/projectors, and particularly slotted cylinder projectors.
2) Description of the Prior Art
It is known to provide slotted cylinder projectors and piezoelectric transducer assemblies. FIG. 1 shows a cross-sectional view of one such prior art transducer. A slotted cylinder transducer 10 features a hollow support member 12 in a cylindrical configuration and having an axial opening 14. The diameter of the support member 12 is D. Transducer material 16 is supported concentrically within the support member and is typically of a piezoelectric material and provided with an axial opening. The outer support member 12 may be thinned at selected locations to facilitate control of vibrational frequency and frequency bandwidth of the transducer assembly. The thinned portions of the support member 12 may be adapted to retain a compliant material, such as urethane, to smooth the outer surface of the support member 12. The hollow interior 14 of the transducer assembly 10 may be filled with a compliant material, such as urethane. The support member 12 is provided with an axially extending side opening 18, and the transducer material 16 is similarly provided with an axially extending side opening 20, the two side openings 18 and 20 being aligned with each other. Side openings 18 and 20 give a slot having dimension “g”. Bending nodes 22 and 24 occur in the transducer 10 opposite side openings 18 and 20. Transducer 10 bends as shown at arrows 26. The slotted cylinder projector operates in a bending mode in a manner analogous to other resonant objects forced by piezoelectric components. These include tuning forks and vibrating cantilevers.
Piezoelectric material must be polled before it can be used as a transducer. Polling involves raising the temperature of the material and putting an electric field across the material in the same direction that a field will be applied to the material in use. When the piezoelectric strain is desired in a different dimension from the direction of electric field application and polling, the transducer material is known as a 3-1 transducer material. In a 3-3 piezoelectric material, strain is produced in the same direction as the polling direction and application of the electric field.
When electrical signals are introduced to the transducer material 16, the transducer material 16 vibrates. The outer support member 12 limits the amplitude of the vibrations of the transducer material 16. Such transducers 10 are generally referred to as slotted cylinder projectors and are capable of providing low frequency acoustics. Slotted cylinder projectors are efficient and small in size, and provide sufficient power to find application in underwater sonar projectors.
The resonant frequency (Fr) of a slotted cylinder projector is proportional to the square root of Young's modulus, Y, of support member 12:
                                          F            r                    ≅                      .0655            ×                          ct                              D                2                                                    =                  .0655          ×                      t                          D              2                                ⁢                                    Y              p                                                          (        1        )            wherein c is sound speed, t is thickness, D is the diameter of the inner ring, Y is the effective Young's modulus, and ρ is the effective density of support member 12.
An equivalent circuit model developed based upon kinetic and potential energies of a slotted cylinder of length L, effective density (ρ), effective Young's modulus (Y), length of the cylinder L, thickness t, diameter of the inner ring D1 wherein M=dynamic mass and KE=stiffness, comprises:M=5.4ρLtD, andKE=0.99YL(t/D)3  (2)
FIG. 2 shows a prior art transducer 30 known as a bender bar joined to a typical electrical driver 32 represented by an alternating current voltage source. Bender bar 30 includes a flexible bar 34 having a transducer member 36A and 36B positioned on either side of bar 34. First electrodes 38A and 38B are positioned on a first side of each transducer member 36A and 36B, and second electrodes 40A and 40B are positioned on a second side of each transducer member 36A and 36B. Insulation 42 is provided to insulate flexible bar 34 from electrodes. As shown, transducer member 36A is poled in the opposite direction from transducer member 36B. The contraction and expansion of transducer members 36A and 36B causes flexible bar 34 to bend in response thereto. When subjected to a voltage from electrical driver 32, this different poling causes transducer member 36B to contract when transducer member 36A expands resulting in bending shown at 44B. When the voltage is reversed, bending reverses to that shown at 44A. Rapidly changing the applied electrical signal causes vibrations in the bender bar 30.
Acoustic transducers and more particularly slotted cylinder projectors are often used in high pressure environments and environments with varying temperatures. These environmental conditions change the resonance frequency of the transducer and cause the transducer to become inefficient and mismatched to its power amplifier.